1. Field of the Invention
This invention relates to a spindle-drive motor having a magnetic fluid seal comprised of precision bearing and lamination elements for proper positioning and sealing.
2. Discussion of Related Art
The use of such magnetic fluid seals in spindle-drive motors for use in hard drives is known. Their function is to seal the actual clean room, in which the storage medium, that is, the hard disk, rotates with respect to the bearing system of the driving motor. According to the related art, these magnetic seals consisting of a circular magnet and circular yoke laminations mounted on one or both sides are supplied in prefabricated form and installed in the motor, and only in one of the last steps of production are they filled with the magnetically conducting fluid supplied with them.
Because of the three-dimensional magnetic field which develops, the fluid introduced completely fills the concentric gap between the shaft and the magnetic seal. This results in a self-contained, largely homogeneous fluid ring which prevents any passage of particles and/or mass exchange between the bearing system and the clean room area of the drive.
Furthermore, it is known that an improvement in the magnetic fluid seal can be achieved by providing an additional flux concentrating lamination which overlaps radially with the yoke laminations between the two magnetic yoke laminations mounted concentrically on both sides of the magnet. The outside diameter of this disk-shaped flux concentrating lamination is smaller than the inside diameter of the disk-shaped ring magnet. The desired radial overlap is achieved due to the fact that the inside diameter of the magnetic yoke laminations is much smaller than the inside diameter of the ring magnet.
This magnetic field seal is also supplied in prefabricated form, with the flux concentrating lamination being permanently enclosed between the magnetic yoke laminations because of the radial overlap with the latter. This new magnetic fluid seal can be filled with the required amount of fluid at the time of fabrication due to the displacement of the fluid ring which provides the seal in the space between the two magnetic yoke laminations and the fact that the flux concentrating lamination is permanently enclosed. This eliminates a process step in final assembly of the motor which leads to lot of rejects.
In addition, the self-contained sealing fluid ring is additionally stabilized between the magnetic yoke laminations due to the centrifugal forces occurring due to rotation, so that leakage and escape of fluid into the clean room is practically impossible. Due to the radially overlapping arrangement of the flux concentrating lamination, the opposing faces of the flux concentrating lamination and the yoke laminations are maximized, but the distances are minimized. The two factors together yield a much lower volume resistance than in the related art when filled with a fluid that also conducts electricity.
However, a disadvantage of this known magnetic fluid seal is that it is very difficult to position this seal and its components accurately on the proper faces in the spindle-drive motor. Because of the low axial distance between the yoke laminations and the flux concentrating lamination, functionally proper positioning of the magnetic fluid seal in the spindle-drive motor is of central importance. This is true in particular of the axial positioning of the flux concentrating lamination between the yoke laminations. The flux concentrating lamination should be positioned as symmetrically as possible between the two yoke laminations, so that the unavoidable axial wobble between the parts rotating relative to one another does not result in contact.
Therefore, a primary purpose of this invention is to improve upon a spindle-drive motor having a magnetic fluid seal of the type described above so that the magnetic fluid seal can be introduced into a spindle-drive motor with a high precision and in cost-effective manner.
First, it should be pointed out that it is assumed for the sake of simplicity that the flux concentrating lamination is mounted on the stationary part of the motor, while the magnet disk and the respective yoke laminations are mounted on the rotating part. However, this invention also includes the kinematic inversion wherein the flux concentrating lamination is mounted on the rotating part and the magnet disk and the yoke laminations attached to the magnet disk are mounted on the stationary part.
Furthermore, this invention includes both spindle-drive motors having a stationary shaft as well as those having a rotating shaft, that is, a shaft connected to the rotor.
Nor should the number or position of magnetic fluid seals used in the spindle-drive motor be understood in a restrictive sense. It is possible to provide only a single seal or several seals may be provided. In the case when two seals are provided, for example, one may be arranged on the upper end of the shaft and the other on the lower end of the shaft. However, more than two seals may also be used.
An important feature of this invention is thus that the flux concentrating lamination (as the part on the inside radially) of the magnetic seal is positioned axially on faces of the spindle-drive motor that are machined to a high precision, and in a preferred embodiment according to this invention, such faces are the faces of the inner bearing ring of the bearing used. It does not matter here whether the face of the inner bearing ring of the bearing beneath it is in direct contact with the flux concentrating lamination of the magnetic seal, or whether there is also a spacer disk between them (indirect contact) which is also machined to a high precision and forms the connection and the support element between the flux concentrating lamination of the magnetic seal and the highly precision machined end face of the inner bearing ring of the bearing. In other words, this spacer disk may be omitted, and instead the flux concentrating lamination may rest directly on a face of the inner bearing ring. This inner bearing ring may be designed in the form of a shoulder which is extended axially upward with the flux concentrating lamination resting on it.
It has been found that reference faces on the shaft are essentially unsuitablexe2x80x94without special measuresxe2x80x94because the shaft is subject to a high tolerance with regard to axial positioning faces in construction and installation. This means in particular that there is an unwanted axial height tolerance on the shaft, which is the sum of different individual tolerances in the axial direction of the shaft, so that ultimately the positioning of a magnetic fluid seal on a corresponding reference face of the shaft leads to an unacceptable installation tolerance - unless other additional measures are implemented (to be described below). Then a flux concentrating lamination centered on this shaft would no longer project centrally between the two rotating yoke laminations of the magnet disk and would not run centrally.
An important aspect of the present invention is providing that instead of this, faces of the inner bearing ring which can be machined to a high precision and can be positioned much more accurately in the axial direction in assembly are used for centering the flux concentrating lamination. In the installed state, this is a prestressed bearing (or several such bearings) where the displacement from the inside ring to the outside ring as a result of the prestress due to the precisely defined play between the rolling elements and the raceways can be calculated exactly and therefore can be predicted accurately. Thus the allocation of the two. bearing rings is very precise and is subject to very little tolerance. If the outer part of the magnetic seal rests on the outer bearing ring, it is thus also possible to center the inner flux concentrating lamination with respect to the outer part of the magnetic seal by letting the flux concentrating lamination rest on the inner bearing ring. Thus, the allocation and axial positioning of the respective parts of the magnetic fluid seal are achieved through the inner and outer bearing rings. If the inner bearing ring is mounted on the shaft, for example, by gluing, at the start of assembly, this fixes the position of the inner ring relative to the shaft unambiguously.
With this invention a high precision system is proposed with the prestressed bearing elements of the bearing and the outer bearing ring, because according to this invention, the radially outer part of the magnetic fluid seal is positioned with a high precision on the outer bearing ring, while the flux concentrating lamination is also positioned with a high precision on the inner bearing ring. Thus, the invention consists essentially of the fact that the radially outer part of the magnetic fluid seal is positioned on the outer bearing ring while the radially inner part of the magnetic fluid seal is positioned on the inner bearing ring.
Direct positioning or indirect positioning can be accomplished by means of a spacer, a spacer disk, elongated parts of the inner and outer bearing rings which are inserted with a high precision.
In an embodiment of the present invention, the radially outer part of the magnetic fluid seal does not rest on the outer bearing ring but instead it rests on axial and radial faces of the rotor which are allocated accordingly (lift). Although an ideal allocation of the inner part of the magnetic fluid seal to the radially outer part is not achieved here, as described in the above mentioned embodiments, it may nevertheless be sufficient for various applications to use the face of the rotor, which is subject to a somewhat greater tolerance, for the radially outer part of the magnetic fluid seal.
In a third embodiment of this invention, a corresponding radial reference shoulder of the shaft may also be used as the seating face for the inner part of the magnetic fluid seal. It is assumed here that the inner bearing ring is in tight contact with the lower face of this radial shoulder of the shaft with no play, and likewise the radially inner part of the magnetic fluid seal (the flux concentrating lamination) rests on the upper face of this radial shoulder of the shaft without any play. This radially projecting ring shoulder then functions as a spacer in the axial direction. It thus serves as an axial spacer which, in this embodiment, is no longer connected to the inner bearing ring of the bearing, but instead is connected to the shaft itself.
Of course, the present invention is not limited to the fact that the outer part of the magnetic seal is in direct contact with a respective face of the outer bearing ring. Again in this case, a corresponding spacer disk, or some other spacer means that is machined to a high precision, may be provided between the outer bearing ring and the magnetic seal.
A preferred type of assembly according to this invention provides for the end face of the lower yoke lamination of the rotating part of the magnetic seal to be brought in contact with the outer ring of the bearing which is also rotating and to be secured axially in this position by means (for example, adhesive) which are essentially known. At the same time the flux concentrating lamination is kept at a distance axially from the inner ring of the bearing which has already been installed by means of a high precision spacer disk, thereby positioning the flux concentrating lamination almost exactly at the center between the two yoke laminations. This position is also secured permanently by essentially known means.
Another solution to this problem has an inside ring which is lengthened partially upward axially as part of the upper bearing on which the flux concentrating lamination is in contact and secured with an end face. The above-mentioned spacer disk is eliminated here and the occurrence of axial wobble is reduced due to the accuracy of the position of the bearing rings.